<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .absolute {
            width: 100px;
            height: 100px;
            line-height: 100px;
            text-align: center;
            background-color: #2486ff;
            border: 1px solid #000;
            position: absolute;  /* 没有父元素 就是相对于浏览器 */
            bottom: 10px;
            left: 10px;
        }

        .fixed {
            width: 100px;
            height: 100px;
            line-height: 100px;
            text-align: center;
            background-color: #FFEE88;
            border: 1px solid #000;
            position: fixed;
            bottom: 10px;
            right: 10px;
        }

        .book {
            width: 700px;
            height: 500px;
            margin: 10px;
            border: 1px solid red;
        }
    </style>
</head>
<body>
<div class="book">book</div>
<div class="book">book</div>
<div class="book">book</div>
<div class="book">book</div>
<div class="book">book</div>
<div class="book">book</div>
<div class="book">book</div>
<div class="book">book</div>
<div class="absolute">绝对</div>
<div class="fixed">返回顶部</div>
</body>
</html>